1. Field of the Invention
The apparatus and process of the present invention relate to filtration of fluids. More particularly, the apparatus and process of the present invention relate to filtering of suspended contaminants in a fluid utilizing pre-coat filtration.
2. General Background of the Invention
When producing oil wells, it is common practice to pump water downhole in order to force the oil out of a formation. When producing as well offshore, it is common practice to pump seawater downhole for this purpose. Organisms in the seawater, however, tend to plug pores in the formation such that, after some time, it becomes impossible to pump any more water into the formation, and consequently impossible to pump any more oil out of the formation.
To overcome this problem, various filtration methods have been used to filter the seawater before pumping it downhole. One method is the use of sand filters. While sand filters are effective at filtering out the organisms, water moves slowly through them. Consequently, a large number of large sand filters are needed to provide enough water for a typical offshore well.
In another filtration method, fluids are filtered through a layer of filter media, which normally comprises diatomaceous earth or perlite. The layer of filter media is normally formed on the upstream side of a filter element in a filtration tank. This process is commonly known as pre-coat filtration. As the suspended contaminants saturate the filter media layer, a pressure differential across the filter media layer and the filter element builds up. The filter elements generally comprise a layer of cloth or screen to prevent the filter media from migrating through the filter element; the maximum value of the pressure differential which can build up across the filter media layer and the filter element is limited by the strength of the screen or cloth to maintain the integrity of the opening size therein. If the pressure differential rises above the maximum value, filter media passes through the filter element with the filtrate, impairing the quality of filtrate. When a predetermined pressure differential is reached, the filter elements are cleaned. Cleaning is typically achieved by back-washing or sluicing the filter media layer from the filter elements, during which time the filter media and the contaminants are discarded. This method requires a new supply of virgin filter media for each filtration cycle. The relatively large amounts of filter media used in this practice generally result in it not being economically feasible and/or operationally practical to utilize pre-coat filtration, due to initial costs, storage problems and disposal costs. The mixture of used filter media and fluid is relatively heavy and bulky to store until it can be properly disposed of (regulations of most countries prohibit dumping of the material into the sea).
There are known processes for what is known as dry cake discharge. This refers to a method of drying the layer of filter media before removing it from the filtration vessel, which makes it lighter and therefore easier to dispose of. All such processes known to applicant require scrapers or vibrators to free cake from the filter elements. The scrapers and vibrators are very costly to maintain, tend to cause mechanical damage to the filter elements, and even when properly maintained do not remove the cake completely.
Often, when time is more important than economy of materials, a method known as body feed filtration can be used. In body feed, a pre-coat layer is formed on the filter elements. Fulid to be filtered enters the vessel and exits through the filter elements. Filter media is continuously added to the filter from a slurry tank before the fluid enters the filtration vessel. The rate of flow of the filter media into the fluid is controlled such that the layer of filter media does not become clogged by the contaminants, but rather the contaminants are spaced enough apart in the layer of filter media that flow through the filter media is still possible. Filtration continues until such time as the layer of filter media is either so thick that fluid cannot easily pass through it, or adjacent layers of filter media are near the point they would come into contact. At that time, the filter media is discharged from the vessel. Although this method can prolong the time interval between cleaning filter elements, it uses even more filter media than standard pre-coat filtration; consequently there are higher initial costs, storage problems, and higher disposal costs for the filter media used in this method.
Attempts to allow the pressure differential across the filter media layer and the filter element to increase while still providing high quality filtrate have also been made (because the pressure differential continually increases during filtering process, a filtration system which can withstand a higher pressure differential across the filter media layer and the filter element will need to be cleaned less often, and consequently will produce less waste filter media to filter a given amount of water). One such attempt includes using, as the filter element, a perforated cylindrical support core and windings of strand material across the support core. This method is disadvantageous in that the filter media often becomes trapped in the strands, making cleaning of the filter elements difficult.
In chemical and food processing plants, a given filtration system may be used at different times to filter different products. Usually, one batch of product would ideally be processed in its entirety, before the next batch of a different product is processed in the same filtration vessel. With current techniques, when it is time to filter a different product in the filtration vessel, the fluid which remains in the filtration vessel is removed through a valve in the lower portion of the vessel, without passing through the filter elements. This portion of the product batch would have to be disposed of (lost) or stored until the next batch of that same product is processed. The nature of the product may not permit storage or, when many different products are processed, may require a large and costly storage facility.
Filtration methods utilizing pre-coat filtration and pre-coat filtration in conjunction with body feed are known collectively as filter aid filtration. Additional information on filter aid filtration methods can be found in U.S. Pat. Nos. 4,388,197 and 4,560,483.